Research On Thermal Dynamic Characteristics And The Second Startup Mechanism Of Ignition In Heavy Oil Reservoirs

Although the energy market has been fluctuating,the demand for oil has been increasing and unconventional energy heavy oil with huge reserves has gradually become the focus.As the main way of heavy oil exploitation–cyclic steam stimulation is close to its economic limit,in-situ combustion?ISC?becomes one of the most potential heavy oil development methods because of its unique engery increasing and viscosity reduction.The key to heavy oil fireflooding lies in the successful ignition of the oil layer,and the successful ignition process of the oil layer depends on the ignition temperature of the heavy oil and the unhindered oil release.For the heavy oil reservoirs which have been developed with cyclic steam stimulation,although the formation temperature increased,the watercut of the reservoirs near the wellbore is very high,the components of crude oil becomes heavier,and the reservoir heterogeneity is more serious after multicycles of steam stimulation.So,it is very important for the reservoirs to achieve rapid and high temperature ignition.The field test of ISC in China is relatively later than that of foreign countries,so,there are still many gaps and shortcomings in the high temperature ignition theoretical research and application research,and it lacks the accumulation of relevant theories and supporting technology research.Therefore,it is necessary to carry out relevant research.This paper first described and evaluated the development and geological characteristics of the heavy oil reservoir block for ISC,clearing the production problems that need to be solved in the process of research.Then,the characteristics of porous media and the fluid in porous media were analyzed,and the problems of thermal-fluid-solid coupling effect of porous media on the second startup process were put forward.In order to clarify the influencing factors and influence degree of low temperature oxidation?LTO?on heavy oil ignition at high temperature,and improve the rate of LTO of heavy oil,the thermodynamic characteristics of LTO of heavy oil was systematically studied.The results of the study showed that,at 190?²40?,non-hydrocarbon deposition was easy to occur during the LTO process of heavy oil,and when the temperature was higher than 260?,the crude oil was on fire.Based on the experimental research,the orthogonal test of variance analysis was used to determine the influence order of the LTO heat release rate of heavy oil:montmorillonite>air to oil ratio>30%water content>temperature.According to the effect of the crude oil ignition process of reservoir factors and operation parameters,the thermodynamic characteristics of ignition process of the heavy oil was further investigated.For this purpose,a high temperature and high pressure self-ignition point test device has been developed,which can be used to monitor the parameters of the thermal dynamic characteristics.Application of this device,the effects of different temperature fractions,different heating temperature,air to oil ratio,clay minerals and water saturation on the heavy oil ignition process were investigated.The results showed that the low temperature fractions played an important role in reducing the ignition temperature.Based on the experimental research,it was considered that clay was regarded as the determinative factor of reducing the IT,heating temperature was regarded as the determinative factor of shortening the ignition time and air/oil ratio was regarded as the determinative facto on improving the gnition effect.Based on the study of the factors affecting the low temperature oxidation and ignition process of heavy oil,the mechanism of ignition process at the second startup for heavy oil reservoirs was systematically studied.The ignition model of crude oil under reservoir conditions was established based on the thermodynamic parameters of the ignition process of different fractions and its thermodynamic characteristics from LTO to HTO process.The suction model of heterogeneous porous media reservoir was established by grey correlation theory.The ignition location was determined according to the distribution of reservoir fluid,and the ignition mechanism of heavy oil reservoir was revealed.According to the capillary force model based on different fractions,the reason for the plugging of porous media,and the mobility analysis of the heavy fractions,measures to improve the heavy oil drainage of heavy oil reservoirs at the second startup stage were put forward,which provided a theoretical basis for the second startup of the heavy oil reservoir ignition.According to the research results,the high temperature ignition technology of heavy oil reservoirs was optimized.Pre-injection air and LTO were used to make the fuel fully exposed to oxygen,at the same time,reduce the water saturation of the near wellbore oil layers,provide the space for the mixing of oil and air in the near well zone,and establish the connected channels between injection and production wells.According to the second start-up mechanism,the ignition position,the heating temperature and the heating time were designed,which made the oil reservoir rapidly reach the high temperature combustion state.After the implementation of high temperature ignition process,the reservoir temperature was raised by28.7?,and the water was pushed to the production well after the pre-injection air stage;and the CO₂ content was 12%¹5%and the H/C ratio was 1³ after high temperature ignition.The combustion state reached the high temperature oxidation combustion,and the output of the production well was 1.5² times as much as that before the ignition.